Lightbulbs
End of semester grade policy
How many scientists does it take to change a lightbulb?
Undergraduates:
None “Bright light - hurts... must go back to bed”.
Postgraduates:
Funding for a new lightbulb ran out six months ago - will have to borrow
from their parents.
Apple engineers:
Seven. One to change the bulb, six to design t-shirts and new gadgets
Russian military scientists:
It‟s top secret.
Nuclear engineers:
One to install the bulb and six to figure out what to do with the old bulb
for the next ten thousand years
Reminders:
HW 7 online, due Monday 24th at midnight
Extra evening class Wed 26th at 7pm
Check results on CULearn regularly
Reading quiz in a second
Preclass notes for lecture 18 available by Wed
Lecture 17 :
Incandescent lightbulbs
How they work
Why they are inefficient
At end of semester I will:
• Scale the 2 hardest midterms up, so that all 3 have the same class average
• Delete your lowest scaled midterm score.
• Add up all points possible during class to get final score and %
• Based on that %, you are guaranteed the following grade:
> 90 % = A
> 80 % = B
> 70 % = C
> 60 % = D
> 50 % = E
< 50 % = F
I reserve the right to scale everyone’s scores upwards, if after looking back at the
last few years that seems appropriate.
How does a lightbulb work?
Lightbulbs
Use vast quantities of power- ~25% of all power for lighting.
Incandescent (regular) lightbulbs waste 88% of that!!
•
•
•
•
•
How do they work?
What determines color?
Why do they “burn out”?
Why are fluorescent lights are more efficient?
Why physics makes it hard to improve efficiency?
Electric
Current
How does an incandescent light bulb work:
• Electric current flows through filament
• Filament gets hot
• Hot filament emits EM radiation
• Electrical energy  EM radiation energy
Physics:
• Introduction to electromagnetic radiation (light and other stuff)
• EM radiation emitted by all objects
- Spectrum – range of colors
- Power – Stefan Boltzman Law
What can we see coming out of a lightbulb?
We can see white(ish) light:
- Visible light is one type of electromagnetic radiation
- Form of energy
- Consists of many different colors – what‟s a color?
Electromagnetic radiation
EM radiation travels in waves, like sound, waves on a string etc
Recall: sound is periodic modulation of air pressure
Sound wave
prism
White light is made up of many different colors.
There are other „colors‟ emitted that we can‟t see
Pressure
incoming light beam
Distance or time
1
Electromagnetic radiation
What is Electric field?
• EM radiation also travels in waves, like sound
• But light can travel through a vacuum so what exactly is „waving‟?
• EM radiation is a periodic modulation of “electric field”
• Light is periodic modulation of “electric field”
Strong field
Electric field
Strong field
distance
Electric field
l
distance
l
Electric field
l
distance
• Color of light depends on its wavelength.
• Electric field exists everywhere in space
• Describes the force on a charged particle at each point in space
• Vector – has a magnitude and direction
• Units: Newtons/Coulomb
• Contains energy
• Created by charges (and created in other ways)
• Analogy: Like gravitational field describes the force on a particle with mass
• Balloon demo
The electromagnetic spectrum
• All EM radiation is a periodic
modulation of an electric field
• Visible light is just one part
of the electromagnetic
spectrum with specific range
of l (and f)
• Visible light is the strongest
part of the EM spectrum
emitted by the sun
See this
next week:
Electromagnetic radiation
Light travels in waves, like sound
Speed of sound, in air
= 330 m/s
Speed of light, in vacuum
= 3 X108 m/s
Frequency of concert A
= 440 Hz
Frequency of red light
= 4.6 X 1014 Hz
Wavelength of concert A
= 0.75 m
Wavelength of red light =
a) 0.75 m
b) 7.5 X 10-5 m
c) 6.5 X 10-7 m
d) 1.38 X 1023 m
• It is the part of the EM that
our eyes can detect
The spectrum of white light?
A spectrometer measures the spectrum (range of wavelengths or frequencies) in light
Understanding a spectrum:
incoming white light beam
Visible
IR
light made up of many colors
Visible
UV
White light
detected power
• Defined as the spectrum of
EM radiation emitted by the sun
• All visible l present with roughly
equal intensity
• LED demo
detected power
IR
UV
For this spectrum, rank power of
light at each color:
a. C greater than B greater than A
b. A greater than B greater than C
c. C greater than A greater than B
d. B greater than A greater than C
e. Cannot tell from this data.
An object is giving off light with this
spectrum. What color in the emitted
light has the most power?
B
A
C
Wavelength
a.
b.
c.
d.
e.
IR,
Yellow/green
UV,
red,
blue,
detector angle
(wavelength or color)
2
Blackbody spectrum
Blackbody spectrum and temperature
• Everything that has a non-zero temperature emits EM radiation
•The spectrum of EM radiation coming from a black object is called the
“blackbody spectrum.”
Look at light bulb with variac to control how much electrical power goes
into it.
If I put half as much electrical power into it, what will happen?
• Go to the blackbody spectrum simulation
a. color will change, get whiter, brightness decrease
b. color will stay the same, brightness decrease
c. color will get redder, brightness decrease
d. color will get redder, brightness the same
e. color will get whiter, brightness the same.
• BB spectrum determined by temperature only.
•The temperature of the object affects both
- The total power of EM radiation emitted by the object
- The range of wavelengths emitted (the spectrum)
How does temperature affect emitted power (brightness)?
Stefan-Boltzman law gives total electromagnetic power
(energy/second) out of a hot object at temperature T
Power = e X  X T4 X a
Area of surface
Temperature of object (in Kelvin!)
Two burners on the stove are at the same temperature,
but the left-hand burner has twice the area. How
much more infrared radiation is it putting out?
a)
b)
c)
d)
e)
The same amount
Twice as much
Half as much
Four times as much
Sixteen times as much.
Power = e X  X T4 X a
Stefan-Boltzmann constant,
 = 5.67 x 10-8 J/(s m2 K4)
e = “emissivity”; how well the light gets out
The Kelvin Temperature Scale: Links T to motion
Comments on Stefan-Boltzman Law
Power = e X  X T4 X a
1. Emissivity (e)
• Property of surface,
• Scale of 0 to 1.
• 1 (BB) means radiates well, 0 is lousy.
area, a
e = 0.9
same T so same color.
2. T in Kelvin (= T Celsius +273)
Room temp
area, a
e = 0.3
65F
18C
291K
All motion
stops
3. Emitted power rises very quickly with temperature
• 1 degree Kelvin = 1 degree Celsius = 9/5 degree Fahrenheit
• 0 degree Celsius = 273 K
3
A particular light bulb‟s filament is at 2000 C.
What is its temperature in Kelvin?
A particular light bulb‟s filament is at 2000 C.
What is its temperature in Farenheit?
a)
b)
c)
d)
a)
b)
c)
d)
2000 K
2273 K
1727 K
2500 K
2000 F
3632 F
1240 F
3246 F
How does temperature affect emitted power (brightness)?
P = eT 4a
 = 5.67 x 10-8 J/(s m 2 K4).
T in Kelvin (= T Celsius +273)
Blackbody spectrum
• Everything that has a non-zero temperature emits EM radiation
•The spectrum of EM radiation coming from a heated object is called the
“blackbody spectrum.” (Although it isn‟t black!)
• Go to the blackbody spectrum simulation
Emitted power rises rapidly
with temperature
 Power at 500K
Power at 450K
700
POWER (W)
600
500
•The temperature of the object affects both
- The total power of EM radiation emitted by the object (P T4)
- The range of wavelengths emitted (the spectrum)
(note: emissivity and coil area will cancel!)
400
300
200
100
0
0
If I raise temperature of heating coil on
stove from 450K to 500K, by what fraction
will power emitted by coil increase?
50 100 150 200 250 300 350
TEMPERATURE (K)
a. Power
b. Power
c. Power
d. Power
e. Power
is 1.11 times larger.
is 250 times larger.
is 1.52 times larger.
is 5.0 times larger.
is 50 times larger
Why are lightbulbs inefficient?
Why are light bulbs inefficient?
Recall: Blackbody spectrum
An efficient light bulb:
Electrical power in
100% conversion
EM energy at visible l
• But…..visible light is only a very small part of
the EM spectrum
• Only 18% EM radiation from bulb is visible
• Rest is IR radiation (heat)
• Go to the blackbody spectrum simulation
Power
UV
Electric
Current
How does an incandescent light bulb work:
- Electric current flows through filament
- Filament gets hot
- Hot filament emits EM radiation
- Electrical energy  EM radiation energy
Visible
IR
Sun: T = 5800C
Max power in visible
where eye works
Light bulb: T = 2500 C
Max power in infrared (IR)
where eye not sensitive
Wavelength (color)
Higher temperature
- More power ( T4)
- Peak power at shorter wavelengths
- Lightbulb filament isn’t hot enough to efficiently produce visible light
4
Identify spectrum of radiation given off by person (37 C)
and a block of barely frozen ice (0 C).
Power
The funny shape of the black body spectrum
The shape of the spectrum for the thermal radiation of an object is rather complicated, but
with some effort it can be understood and a formula derived that describes it. That is too
advanced for this class, but in case you are interested, the formula for the amount of
power at each frequency  of an electromagnetic wave is given by
P = (8h/c3)3 (eh/kT -1)-1, where k is the Boltzmann constant (= 1.38 x 10-23
Joules/Kelvin), c is the speed of light (3 x 108 m/s), h is Planck’s constant (6.626 x10-34
J∙s), and the temperature T is in units of Kelvin. From this equation it is possible to show
that the wavelength where the power is highest is given by lpeak = hc/(2.82 kT)
3
detected power
1
l peak = hc/(2.82 kT)
2
 shifts to shorter wavelength
as T increases.
wavelength
a typical spectrum
See spectral simulator for
homework
Wavelength
a.
b.
c.
d.
e.
2 is person, 1 is ice,
1 is person, 2 is ice,
None, because ice gives off no thermal radiation,
1 is both ice and person because they are almost identical
3 is person, one of the others is ice.
Thermal imaging
This remote temperature sensor works by measuring
the infrared spectrum of the thing I point it at.
• Look at BB spectrum for a person (~ 313 K)
• No visible radiation emitted (too cold) – all in the IR
• Not detected by the eye, but detected by an IR (thermal imaging) camera
Thermal imaging – photography using non-visible EM radiation
Try it out with hot plate
5